It is known that a liquid crystalline polyester is a polymer comprising a rigid molecular chain, and highest strength and elastic modulus can be obtained among fibers prepared by melt spinning by highly orienting the molecular chain in the fiber axis direction in the melt spinning and further carrying out a heat treatment (solid phase polymerization). Further, it is also known that the liquid crystalline polyester can be improved in thermal resistance and dimensional stability by solid phase polymerization because the molecular weight increases and the melting point elevates by solid phase polymerization (for example, Non-Patent document 1). Thus, in a liquid crystalline polyester fiber, a high strength, a high elastic modulus, and excellent thermal resistance and thermal dimensional stability are exhibited by carrying out solid phase polymerization.
In the liquid crystalline polyester fiber, however, because the rigid molecular chain is highly oriented in the fiber axis direction and a dense crystal is produced, the interaction in a direction perpendicular to the fiber axis is low, fibril is liable to occur by friction, and there also be a defect that the fiber is poor in abrasion resistance.
Further, for the solid phase polymerization of liquid crystalline polyester fiber, a process for forming the fiber as a package and treating it is industrially employed from the points of simplifying the apparatus and improving the productivity, but, in this process, there is a problem that a fusion between single fibers is likely to occur in a temperature region where the solid phase polymerization can proceed and there occurs a defect due to a delamination of the fused portion when unwound from the package. Such a defect impairs the uniformity in the fiber lengthwise direction causing a reduction of strength, and in addition, causes a problem of fibrillation of the fiber proceeding from the defect as an origin.
Recently, particularly for a filter made of monofilaments and a gauze for screen printing, requirements of densification of weave density (making a mesh higher), decrease of thickness of the gauze and making an opening have a large area are increased for improving the performance, and in order to achieve this, making the single fiber have a small fineness and a high strength is strongly required, and at the same time, decreasing the defects of the openings is also required for providing a high performance. For decreasing the defects of the openings, because the aforementioned fibril is produced by fusion defect in the solid phase polymerization or friction in a higher-order processing, it is required to increase the strength and the uniformity of the fineness in the fiber lengthwise direction and to improve the abrasion resistance of the fiber.
Moreover, deterioration of a process passing-through property at a fiber higher-order processing process such as weaving is caused by engagement of fibril or fluctuation of tension due to accumulation of fibril onto a guide, and also from this point, it is required to increase the strength and the uniformity of the fineness in the fiber lengthwise direction and to improve the abrasion resistance of the fiber.
With respect to improvement of the abrasion resistance of liquid crystalline polyester fiber, a core-sheath type compound fiber in which the core component comprises a liquid crystalline polyester and the sheath component comprises a polyphenylene sulfide (Patent document 1) and a sea-island type compound fiber in which the island component comprises a liquid crystalline polyester and the sea component comprises a bendable thermoplastic polymer (Patent document 2) are proposed. In these technologies, although the abrasion resistance can be increased by the bendable polymer forming the fiber surface, there are problems that the strength of the fiber is poor because the percentage of components other than the liquid crystalline polyester is great, and that the fiber surfaces with a low melting point are fused with each other in the solid phase polymerization required for making the strength of the liquid crystalline polyester greater and defects are likely to occur. Further, in the core-sheath type compound spinning such as one in Patent document 1, each of the discharge amounts for core and sheath is little as compared with that for a single-component spinning, and when the discharge amount is further decreased in order to make the fiber fineness smaller, the melt viscosity changes by gelation or thermal decomposition accompanying with increase of residence time, irregularity in fineness or abnormal compounding occurs in the fiber lengthwise direction, and therefore, the uniformity in the lengthwise direction is impaired. Further, also in the blend spinning such as one in Patent document 2, when the discharge amount is decreased in order to make the fiber fineness smaller, an influence of blend irregularity in the lengthwise direction is actualized, and therefore, the uniformity in the lengthwise direction is impaired.
Further, a technology is proposed wherein the abrasion resistance is improved by heat treating a compound fiber comprising a liquid crystalline polyester and a bendable thermoplastic resin at a temperature of the melting point of the bendable thermoplastic resin plus 20° C. of higher (Patent documents 3 and 4). In this technology, however, because the abrasion resistance is improved by turning the bendable thermoplastic resin into an amorphous state, there is a problem that the obtained fiber is poor in thermal resistance. Further, because of compound fiber, as aforementioned, there is also a problem that the uniformity in the lengthwise direction is impaired.
These problems are ascribed to the means of compounding of a liquid crystalline polyester and the other component, and from this point, a technology has been desired for simultaneously achieving a small fineness, a high strength, a high uniformity in a lengthwise direction and a high abrasion resistance by a single component of liquid crystalline polyester.
With respect to improvement in abrasion resistance of a single-component yarn, in a polyamide, polyvinylidene fluoride or polypropylene monofilament for a fishline, a fishing net or a mower, a process is proposed wherein the abrasion resistance is improved by adding heat more than the melting point to a monofilament after stretching and accelerating the relax of orientation of the surface layer (Patent documents 5-9). However, this technology is a technology capable of being achieved by the condition where the polymer is a bendable polymer and therefore the time required for the relax of orientation (relax time) is short, and in case of rigid molecular chain such as that of a liquid crystalline polyester, the relax time becomes long, there is a problem that the inner layer is also molten within the relax time for the surface layer and the fiber is molten. Moreover, as the single-fiber fineness becomes smaller, the influence due to the heat treatment reaches a central portion of the fiber, and therefore, there is a problem that it is difficult to achieve both of sufficient strength and abrasion resistance.
Further, a technology is proposed wherein, after a liquid crystalline polyester fiber is heated and cured at a temperature lower than the melting point (solid phase polymerization), it is stretched at 10% to 400% within a range of 50° C. from the curing temperature to increase the strength and the elastic modulus (Patent document 10). However, this technology aims to further enhance the orientation of the molecular chain by stretching at a temperature capable of maintaining the crystallinity and to increase the strength and the elastic modulus, and because the fiber structure is high in degree of crystallization and high in orientation of molecular chain, the abrasion resistance cannot be improved. Where, in this technology, although the relationship between the stretching temperature and the melting point of the liquid crystalline polyester fiber served to the stretching is shown only in its Examples 3 and 4, the stretching temperature is lower than the melting point of the liquid crystalline polyester fiber, and an advantage by heating a solid phase polymerized liquid crystalline polyester fiber up to the melting point or higher is not suggested at all.
Furthermore, a process is proposed wherein, in order to increase the abrasion resistance of a liquid crystalline polyester fiber, polysiloxane and/or fluorine-group resin are adhered to the fiber surface and dried at 100° C.-300° C. or calcined by heating at 350° C. or higher (Patent document 11). In this technology, however, although a high-temperature treatment is carried out for drying or calcination, this is a treatment for making the adhered polysiloxane and/or fluorine-group resin hard to be left, there is no description on the relationship with the melting point of the liquid crystalline polyester fiber to be treated, and it is not a process for improving the abrasion resistance of the fiber itself by change of the structure.
On the other hand, with respect to giving a liquid crystalline polyester a small fineness, there are two problems of a problem originating from solid phase polymerization and a problem originating from spinning. The problem originating from solid phase polymerization means a problem that, because the specific surface area increases accompanying with making the single-fiber fineness smaller in the solid phase polymerization at a package condition, the contact points between single fibers increase, fusion is liable to occur, and defects increase. The problem originating from spinning means a problem of a poor fiber formation property or an abnormal fineness due to decomposition or deterioration accompanying with increase of residence time in a spinning machine when the discharge amount is decreased, or a problem of a poor fiber formation property or an abnormal fineness due to an instability of forming fiber when the spinning speed is increased.
With respect to suppressing fusion at solid phase polymerization, Patent document 12 proposes a process for heat treating a package wound at a winding density of 0.16-0.5 g/cc. By this, a fusion can be avoided to some extent, but in case of treating a fiber with a low total fineness, the affection due to the fusion cannot be solved. Further, although Patent document 13 describes to control the winding density at the time of solid phase polymerization of a liquid crystalline polyester monofilament with a total fineness of 50 denier (55.5 dtex) or more at 0.3 g/cc or more, it does not describe as to fusion at the time of solid phase polymerization though the reaction efficiency for the polymerization is described.
By the way, with respect to making a modified liquid crystalline polyester fiber, a technology is proposed wherein a liquid crystalline polyester with a specified composition is used, and a high strength can be achieved without solid phase polymerization by melt spinning using a nozzle whose introduction section is formed to be taper (Patent document 14). However, the fineness achieved in this technology is 19 dtex at smallest, and a small fineness for the liquid crystalline polyester with a specified composition cannot be achieved. Further, in this technology, although the strength is high, there is a problem that the thermal dimensional stability and the elastic modulus are poor because solid phase polymerization is not carried out. Further, because the flow line may become unstable by the taper nozzle used in the technology, the fiber formation stability is poor, and although a small amount of samples can be obtained, fiber formation for a long time is difficult, and in particular, when the spinning speed is increased that is important for making the fineness of the fiber smaller, the fiber formation property further deteriorates. Where, although an example having carried out solid phase polymerization is also disclosed in Patent document 12, the single-fiber fineness is 51 dtex and it is thick, and a technology for improving fusion in the solid phase polymerization when made the fiber fineness smaller is not suggested at all. Non-Patent document 1: Edit by Technical Information Association, “Modification of Liquid Crystalline Polymer and Recent Applied Technology” 2006, pages 235-256
Patent document 1: JP-A-1-229815 (first page)
Patent document 2: JP-A-2003-239137 (first page)
Patent document 3: JP-A-2007-119976 (first page)
Patent document 4: JP-A-2007-119977 (first page)
Patent document 5: JP-A-60-231815 (first page)
Patent document 6: JP-A-61-152810 (first page)
Patent document 7: JP-A-61-170310 (first page)
Patent document 8: JP-A-5-148707 (first page)
Patent document 9: JP-A-8-158151 (first page)
Patent document 10: JP-A-50-43223 (second page)
Patent document 11: JP-A-11-269737 (third page)
Patent document 12: JP-A-61-225312 (first page)
Patent document 13: JP-A-4-333616 (fourth page)
Patent document 14: JP-A-2006-89903 (first page)